Radiant energy heating systems are commonly used in industrial and commercial buildings. Radiant heater systems are more efficient and cost-effective than forced air systems because the emitted infrared rays heat objects and concrete floors rather than large volumes of air. The concrete floors absorb the radiant energy and re-radiate it back into the surrounding air to maintain a blanket of warmth within the heated space. By thus heating from the bottom up, the stratification of heat within the building that tends to occur with a forced air system is minimized, with consequent improvements in comfort and fuel efficiency.
Some radiant space heating systems are supplied in component form, the radiator tube of the installed system being built up from various straight and curved sections as needed to route the radiator tube through the area of the building where heating is desired. In such a system, a burner unit is located at a first end of the tube and an exhaust fan is typically located at the opposite end of the tube to draw the hot effluent through the tube. Alternatively, a radiant heater may be manufactured and delivered as an assembled unit comprising an elongated, U-shaped radiator tube, a burner unit, and an exhaust fan. A reflector is disposed above the radiator tube to direct the radiant energy in the desired direction. The unit is provided with hangers which permit it to be suspended from a ceiling or other overhead structure. Such radiant heater units intended for industrial or commercial use and rated at 20k BTU/hour to 50k BTU/hour are typically over 10 feet in overall length.
Scaled down versions of the above-described radiant heater units have been produced and marketed for use in residential heating applications. Such residential units have been six feet or more in overall length, and therefore can not be mounted within the standard two foot by four foot openings in suspended ceiling systems.
If a conventional U-tube radiant heater is scaled down to have an overall length of less than six feet, the total length of the radiator tube is reduced to the point where the exhaust effluent still contains significant useful thermal energy. This leads to a reduction in the efficiency of the heater, and requires that the exhaust ducting be able to withstand higher temperatures.
Another reason why it has not previously been practical to produce a radiant heater in the 20k-40k BTU/hr range having an overall length of four feet or less is that burners as heretofore used to produce heat outputs in that range generate a flame which is well over four feet long. Even using a U-shaped radiator tube, the tube can not have a straight segment more than four feet long, so that the flame must impinge on the inner surface of the radiator tube in the vicinity of the 180.degree. bend. Impingement on the flame on the tube causes quenching of the flame, with a consequent increase in undesirable emissions such as carbon monoxide. The flame also crates a hot-spot on the radiator tube, which results in premature oxidation of the tube material and eventual burn-through.
It is possible to decrease the length of the flame produced by the burner by increasing the amount of air flow through the burner venturi. Conventional methods for achieving this increase in what is known as the primary air flow involve either a) enlarging the diameter of the venturi or b) increasing the pressure of the air drawn or forced through the venturi. Both of these options have effects that are undesirable in a heater unit for residential use.
Increasing the venturi diameter is undesirable because it forces an increase in the diameter of the radiator tube in order to maintain adequate clearance between the exterior of the venturi and the interior of the surrounding radiator tube. Sufficient clearance must be maintained to permit the flow of air around the outside of the venturi, the flow of this so-called secondary air being critical in cooling the radiator tube containing the flame. An increase in radiator tube diameter increases the depth of the unit, and also forces an increase in the radius of the bend connecting the straight segments of the tube so that they must lie farther apart, thus making the unit wider.
The other possibility, increasing the air velocity through the burner, leads to an increase in the level of noise made by combustion, the fast-burning flame making a "roaring" sound. In a residential application it is particularly important that the heater operate quietly.
Another drawback inherent in prior radiant heaters is that electromechanical components of the heater, such as the fan, gas supply valve and electronic control unit, are difficult to service and inspect when the heater is installed in an overhead position. Servicing and/or inspection of these components involves either working on the heater in its installed position, or removing the entire heater from its installed position and moving it to a location where the components are more easily accessed.
Some gas-fired radiant heaters include what is known in the art as an air flow switch, a mechanism for determining the amount of air being provided to the burner and shutting off the flow of gas if the air flow is not sufficient for proper functioning of the heater. Air flow may be reduced by any number of problems, for example a malfunctioning or blocked fan, or a blockage anywhere along the radiator tube or its exhaust ducting.
In a heater having a pressurized plenum chamber supplying air to the burner, the air flow monitoring function is generally accomplished by a pair of pressure actuated switches located within the plenum chamber and wired to a control unit which cuts off the flow of gas through the valve if the pressure switches indicate that the pressure within the chamber is not within the acceptable range. One switch is actuated and sends a flow termination signal to the control unit if the pressure in the plenum chamber drops below a predetermined minimum, such as would occur if the fan were to slow or stop. The other switch sends a flow termination signal if the pressure rises above a predetermined maximum, such as would occur if the radiator tube or exhaust were blocked at any point downstream.
The cost and complexity of the heater could be reduced if the air flow switch function could be accomplished with only one pressure switch requiring a single electrical connection to the control unit.